A document has just gone on display at Mount Vernon, Virginia - the museum in the former home of George Washington, first US President. It is an order dated 1777 and signed by Washington himself to send troops that had not been vaccinated for smallpox - or survived it - to Philadelphia to be vaccinated. These troops were then to join up with the main army, where the disease was raging.

It sounds like amazing foresight for its day. "Washington's careful handling of the smallpox epidemic at the beginning of the war was a significant reason for the disease not decimating his army", says Mount Vernon.

Not quite. Washington's order was likely a response, not just to a normal smallpox epidemic, but to a bioweapon wielded by the British enemy - a strategy that the redcoats had already used against the colonists to great effect earlier in the American revolutionary war.

Historically, disease was always the real enemy of armies - the First World War was the first in which enemy action killed more soldiers than disease did. In 1776, more than half of all people caught smallpox at some point, and a third of those died. Edward Jenner did not popularise the use of the related, milder "cowpox" virus for "vaccination" until 1798.

But pre-Jenner, smallpox itself was used to immunise - a practice called variolation widespread in the American colonies at the time of the revolution. That was what Washington sent his troops to Philly to get. He later set up special clinics to inoculate all new recruits.

While people were variolated in ways that reduced the severity of the infection - only 1 or 2 per cent died - if you caught smallpox from someone still experiencing this mild disease you often got full-blown smallpox. That made people who were recently-variolated a threat to anyone without immunity to smallpox.

Washington's army was largely composed of rural conscripts who were far less likely than city-dwellers to have been exposed to smallpox. And without being safely variolated, they were sitting ducks, not just to normal smallpox, but to other people who had recently been variolated themselves.

That's certainly what the wily British were counting on. In his admirable history of smallpox, biodefence expert Jonathan Tucker (tragically found dead recently at the age of 56) confirms that British troops in North America indeed deliberately spread smallpox to control restive Indians in the 1760s. By 1775, the British defending Boston from the rebels had already inoculated all their troops. When smallpox broke out in the town, they sent recently-variolated civilians among the besieging colonists, causing an outbreak that delayed the eventual American victory.

In 1776, the British did it again, as the Americans besieged Quebec City - the story goes that this time they variolated prostitutes and sent them among the troops. Half the 10,000 Americans fell ill, and after burying their dead in mass graves, Tucker reports, they retreated in disorder from the colony, which remained in British hands (as did their commander, Benedict Arnold, who infamously turned coat). I'm Canadian - I may well owe that to this spot of biowarfare.

So in 1777, Washington was hardly acting before time in getting his troops inoculated for smallpox. The irony, of course, is that now, 30 years after smallpox was eradicated in the wild, fears that someone will use remaining stocks of the virus as a weapon mean the US still vaccinates its troops, and others.

Because of Jenner it's now a safer procedure, though not risk-free. And unlike in 1777, risks from the vaccine now top the risk you'll get smallpox - unless some would-be bioterrorist really does have plans to emulate the British in 1776. If so, please take note: that time, the British lost.

Top image: Everett Collection/Rex Features. This post originally appeared on New Scientist.